Transistor base resistor calculation on 2N4401

[matt99199]

I am having some trouble trying to calculate a base resistor for a 2N4401 transistor.
My problem is, what do I do for an hFE value when the collector current is between the given values on the spreadsheet?
For example, the 2N4401 lists a hFE of 100 for 150mA and 40 for 500mA, can I just draw a linear line betweeen these two points to find the value if I have a collect current of 300mA? or would I just assume a different value?

 

[Arouse1973]

You divide your collector current by your gain. This is your base current. You then take you input voltage and subtract 0.6 and divide this by your base current to give you your base resistor value.
Adam

 

[KrisBlueNZ]

You divide your collector current by your gain. This is your base current.

But the data sheet doesn't specify a minimum h_FE at the collector current he wants to use. He wants to run it at 300 mA and the data sheet has h_FE values for 150 mA and 500 mA but not for 300 mA. That's what he's asking.

I can't give a definitive answer. I think I would use the typical characteristics graph for h_FE. I would take the typical graph and plot the minimum figures from the table in the data sheet, and join them up with a shape that roughly matches the 25 °C typical curve. Then read the minimum gain at whatever current you want. That wouldn't be guaranteed; if you want a real guaranteed minimum, I guess you would have to use the 500 mA value :-(

 

[KMoffett]

Also need to state what you what to use this 2N4401 for. Do you want to operate in a linear mode as in an audio amplifier, or in a switching mode to just change between open and saturation.

Ken

 

[KrisBlueNZ]

Good point. If you're using it as a saturated switch, see:
https://www.electronicspoint.com/resources/using-a-bipolar-transistor-to-turn-a-load-on-and-off.30/
https://www.electronicspoint.com/resources/saturation-in-transistors-bjts-why-and-how.28/

 

[matt99199]

Yea, sorry, I didn't specify that I am using the 2N4401 in a switching mode.
I looked into the data sheet but the graph for hFE vs Ic doesn't extend past 10mA for some reason.

If I do a linear plot from two points, calculate, and then use a real resistor value slightly higher than my calculation, wouldn't that be a safe way to ensure it is not over saturated?

 

[KrisBlueNZ]

If I do a linear plot from two points, calculate, and then use a real resistor value slightly higher than my calculation, wouldn't that be a safe way to ensure it is not over saturated?

Did you read the two resources I linked to?

 

[KMoffett]

A rule of thumb that I use to assure CE saturation for switching transistors is to use base current of 1/10th the expected collector current. So for a collector current of 150mA, I would select a resistor for 15mA base current. As in post #2: Rb=10(Vin-0.6)/Ic

Ken

 

[KrisBlueNZ]

A rule of thumb that I use to assure CE saturation for switching transistors is to use base current of 1/10th the expected collector current. So for a collector current of 150mA, I would select a resistor for 15mA base current. As in post #2: Rb=10(Vin-0.6)/Ic

I think it's probably better to work it out based on the h_FE of the transistor. Divide the collector current by the minimum specified h_FE at that collector current, then multiply by a "fudge factor". That's the way I described it in one of those resources.

A big power transistor may only have a guaranteed h_FE of 20 or less at a high collector current. If you drive it with only 1/10th of the collector current, it may not saturate very well.

 

[mursal]

We use the 10 rule to drive it hard into saturation, when switching ...........
Good point about the bigger transistors not going into saturation, though